1. Field of the Invention
The present invention relates to sizing paper and the application of a release coating to a paper substrate, a nonwoven substrate, or a woven substrate.
2. Description of the Prior Art
A. Saturant Technology
In the manufacture of paper and paperboard from cellulosic material, it is customary to use a sizing agent either at the wet end, known as "internal sizing", or in the drying section of the paper making machine. This is done to increase the resistance of the paper or paperboard to wetting and penetration by liquids, particularly aqueous liquids, and thereby provides the paper product with water resistant properties.
The term "paper" as used in this invention refers to a web of felted or matted sheets of nonwoven cellulose fibers, formed on a fine wire screen from a dilute water suspension, and bonded together as the water is removed and the sheet is dried. Paper also refers to sheet materials produced from other types of fibers, particularly mineral or synthetic, which can be formed and bonded by other means. Of particular importance in the present invention, is the unbleached kraft paper commonly used in the manufacture of masking tapes and packaging tapes. Also of importance are nonwoven fiber or woven fiber substrates from synthetic or natural sources, such as nylon, rayon, cellulose diacetate, cellulose triacetate, polyamide resins, polyester resins, polyacrylic resins, polyvinyl resins, polyolefin resins, glass, metal, abaca, sisal, henequen jute, cotton, ramie, flax, hemp, silk, wool, mohair, cashmere, vicuna, alpaca, and the like.
The process of saturating or impregnating paper or a nonwoven fiber substrate with a resin or polymer is also known in the trade as sizing.
The application of a sizing agent to paper sheet or paperboard in the drying section of the papermaking machine is generally referred to as tub sizing. Partially dry sheet is passed through a size solution or over a roll wetted with size solution to saturate the paper. The same procedure can be applied to a nonwoven substrate but not generally to a woven substrate, which ordinarily has sufficient strength.
The two principal methods of sizing paper are the tub sizing method and off-machine saturation method. In tub sizing, the paper is contacted with the size while still on the paper machine and in a partially dry state. A relatively small percentage of manufactured paper is saturated by this method.
Where paper is used to make adhesive coated tapes, such as masking tapes or packaging tapes, it is necessary to improve its tensile strength and tear resistance by impregnating the paper with a resin or polymer to confer these properties. Also improved is delamination resistance, whereby the tendency to split in the plane of the paper is reduced.
When paper in the form of tape is coated with a pressure sensitive adhesive and the adhesive coated tape is wound upon itself to form a roll, as for example with masking tape or packing tape, it is also necessary to apply to the non-adhesive coated side of the tape, a controlled release coating, also known as "backsize" in order to facilitate the unwinding of the tape from the roll and its use.
If the tape cannot be unwound, or unwound only with great difficulty, it is said to be "blocking". Optimally, the tape must unwind in a controlled fashion so that only the amount desired for use is unwound from the roll. The tape should not be capable of unwinding in an uncontrolled manner when handled, so that more tape separates from the roll than the amount desired.
Suitable adhesives for pressure sensitive adhesive tape applications include tackified rubber adhesive solutions, tackified hot melt adhesives, tackified rubber emulsion adhesives and acrylic ester adhesives.
The majority of manufactured paper is saturated with a sizing composition, known as a "saturant", by means of an off-machine saturation method wherein the paper is saturated on a separate machine and is dry at the time of saturation. The conventional means for saturating paper in the off-machine method is to dip the paper in a dilute latex or resin emulsion, and then pass the paper between nip rollers under high pressure to remove excess resin or saturant. The paper is then passed through an oven to dry and set the resin.
The primary disadvantage of the conventional off-machine saturation method wherein the paper is dipped, and the excess resin is squeezed, lies in the large amount of water which becomes absorbed by the paper. For example, in the saturation of bleached crepe paper, used to produce masking tape, a normal dry add-on of styrene-butadiene-rubber (SBR) latex to achieve acceptable tensile and delamination properties is generally about 10 to 50% by weight of the dry paper.
In order not to exceed this add-on, the latex is usually applied from a dip bath having a solids level of about 25 to 30%. Wet pick-ups from a bath such as this range from about 75 to 150%. "Wet pick-up" refers to the weight of the added substance plus the vehicle, the weight of the web as a basis. Styrene-butadiene-rubber latices are manufactured at about 50 to 55% solids. It thus becomes necessary to dilute the latex with water in order to avoid adding more rubber solids to the paper than is necessary to achieve the desired tensile strength.
All water added must then be removed in drying ovens. Obviously, a significant amount of the energy expended in drying could be conserved if it were not necessary to dilute the latex with water. In addition, the running speed of a saturation range is generally controlled by the drying capacity of the ovens.
B. Release Coatings
When the sized paper is used to produce adhesive coated tapes, a controlled release coating, referred to as "backsize" is generally applied to the surface of the dry, saturated paper stock by coating a low solids polymer solution, such as that of acrylic ester copolymer resins, vinyl acetate copolymer resins, silicone resins, polyamide resins and polyester resins by conventional coating methods such as a kiss roll or a wire wound roll.
Historically, non-aqueous solvent solutions were generally used. However, due to environmental considerations, aqueous emulsions or latices have recently been introduced with much success, displacing the use of non-aqueous solvent solutions.
Controlled release latices are available at a level of about 40 to 50% solids, and are diluted with water to about 10 to 30% solids in order to meter the application of the release coating so that only a sufficient amount is used to achieve the desired release properties.
Unlike the saturant which must penetrate wetted paper, the controlled release coating is applied only on the surface of one side of the paper.
Silicone emulsions are also used for controlled release coatings, and are applied from emulsions containing about 2 to 10% silicone solids, since only small amounts of silicone are necessary to achieve the desired release properties.
As with the saturant, the ability to apply the release coating to the paper surface or other substrate using a high solids content emulsion would result in energy saving benefits in the dryer provided that the wet pickup is proportionately lower.
U.S. Pat. No. 4,571,360 to Brown et al discloses uniformly distributing paper treating agents onto paper without changing the properties of the paper by applying fast breaking and fast wetting foams. Ionic foaming agents were added to distribute the treating agent evenly to the paper. In all the examples starch was the treating agent. Foam may be applied to either side of the paper in multiple or two-sided applications, or sequentially.
U.S. Pat. No. 4,581,254 to Cunningham et al discloses applicators for uniformly distributing treating agents, such as cooked starch, to rapidly moving paper.
U.S. Pat. No. 4,597,831 to Anderson discloses application of water-repelling, external sizing such as rosin to the surface of paper. The rosin used was self-foaming and the use of foaming agents was discouraged.
U.S. Pat. No. 4,184,914 to Jenkins discloses the use of a foamed protein added to paper pulp before its entrance to the mesh of a papermaking machine in order to reduce the amount of water picked up by the pulp. The protein foaming agent also reduces the surface tension of the water.
The brochure "Foam Bonding - Dewtex" published by Rando Machine Corporation, Macedon, N.Y. discloses foam bonding of fabrics employing foamed synthetic rubber and polyvinyl acetate latices as adhesives.
U.S. Pat. No. 4,279,964 to Heller discloses densification of a high solids froth of a resin emulsion and a starch solution, or a frothed starch solution coated onto a paper substrate to increase its opacity and ink hold out.
U.S. Pat. No. 4,288,475 to Meeker teaches vacuum impregnation of a fibrous web by a foamed binder consisting of up to 60% of the weight of the material.
U.S. Pat. Nos. 4,193,762 to Namboodri; 4,118,526 to Gregorian et al; 4,094,913 to Walter et al and the text of a presentation by George C. Kantner, "Frothed and Foam Coatings for Upholstery and Nonwoven Fabric Applications" FOAM TECHNOLOGY IN TEXTILE PROCESSESS, (PD 186-03, July 30-31, 1985), all relate to treating fabrics with a foam composition.